Turbulent and neoclassical transport in tokamak plasmas

Doktorsavhandling, 2011

One of the greatest challenges of thermonuclear fusion is to understand, predict and to some extent control particle and energy transport in fusion plasmas. In the present thesis we consider theoretical and experimental aspects of collisional and turbulent transport in tokamak plasmas.
First the collisionality dependence of quasilinear particle flux due to ion temperature gradient (ITG) and trapped electron modes is investigated. A semi-analytical gyrokinetic model of electrostatic microinstabilities is developed and used to study various parametric dependences of ITG stability thresholds and quasilinear particle and energy fluxes, focusing on the effect of collisions.
Then corrections to the neoclassical plateau regime transport in transport barriers are calculated. It is found that the ion temperature gradient drive of the bootstrap current can be enhanced significantly, and the ion heat diffusivity and the poloidal flow of trace impurities are also modified in the presence of strong radial electric fields.
Furthermore, we investigate the characteristics of microinstabilities in electron cyclotron heated and ohmic discharges in the T10 tokamak using linear gyrokinetic simulations, aiming to find insights into the effect of auxiliary heating on the transport, with special emphasis on impurity peaking.
The effect of primary ion species of differing charge and mass on instabilities and transport is studied through linear and nonlinear gyrokinetic simulations. We perform transport analysis of three balanced neutral beam injection discharges from the DIII-D tokamak which have different main ion species (deuterium, hydrogen and helium).
Finally the magnitude and characteristics of the error in alkali beam emission spectroscopy density profile measurements due to finite beam width are analyzed and a deconvolution based correction algorithm is introduced.

The goal of fusion research is to provide a clean and safe energy option with large fuel resources. Since the fusion reaction requires extremely high temperatures, fusion energy production is a scientifically and technologically challenging problem. The fusion fuel is confined by a magnetic field in a way that the hot region (the “plasma core”) is not in contact with the wall of the device. To operate these devices efficiently we need to understand and keep particle and heat transport between the plasma core and the edge under control.
Transport of particles and heat in fusion devices is caused by collisions between the plasma particles and by turbulent flows in the plasma. Plasma turbulence is driven by small scale modes, so called microinstabilities. In the first part of the thesis we derive a relatively simple, but accurate model to study microinstabilities, focusing mainly on how they are affected by collisions and the fluxes driven by them. In the next part we calculate how the collisional transport is modified by strong electric fields that are present in transport barriers (i.e. regions characterized by strong density gradients and reduced turbulent transport). We also study how heating with electromagnetic waves or the mass and charge of the fuel ions affect microinstabilities and transport. Finally we present a correction method for atomic beam diagnostics measurements that is used to measure plasma density and turbulent fluctuations at the plasma edge.